1. Field of the Invention
The present invention relates generally to environmental testing and, more particularly, to fixtures used in vibration testing.
2. Description of the Prior Art
Manufacturers have observed that devices experience a high failure rate immediately after being placed in service, often referred to as "infant mortality," and as they near the end of their service life, often referred to as "old age." During a device's "mid-life" between infant mortality and old age, failure rates usually drop precipitously to a much lower value than those experienced during infant mortality and old age. Consequently, if before shipping a product a manufacturer can reliably and economically identify and eliminate those particular devices that will experience infant mortality, it can significantly improve the reliability of its products as perceived by customers, while also significantly reducing its warranty expenses. Studies of devices which experience infant mortality have shown that most such devices fail because of a latent defect introduced into the device during its manufacture. However, only after such a device experiences stress in its normal operating environment does the latent defect reveals itself by the device's failure.
Environmental stress screen ("ESS") is a manufacturing procedure applied to devices before they enter service which attempts to establish that they are free of defects. One technique employed in ESS involves securing the device to a vibration apparatus, e.g., a shaker, and then shaking the device while concurrently operating the device and recording its operation. Advanced ESS involves continuously monitoring a device's operation while exposing it both to broadband random vibration and to ultrahigh-rate temperature changes.
ESS testing is itself a process, and, like all manufacturing processes, must be controlled if it is to be applied repetitively and effectively to determine if a product is free of defects. Repetitively and reliably discovering hidden (latent) flaws or defects in materials, process, or design with ESS requires that a device be exposed to uniform high-acceleration vibration. If vibration equipment exhibits large variations, including variations as measured along each of a device's three mutually perpendicular axes, i.e. X, Y and Z axes, it is impossible to assert that the ESS process is both effective at identifying defective devices and does not damage devices being tested.
A shaker used in the ESS vibration process usually includes a table for mounting the device to be tested. That table is, in turn, coupled to a source of vibration, e.g. a pneumatically, electro-magnetically or hydraulically energized driver. Evaluation testing of equipment specifically configured for the ESS process has revealed inconsistencies and large variations in vibration levels across a single shaker table. This ESS equipment, which employs multiple independent vibration sources for simultaneously vibrating a device along each of three axes, exhibits vibration levels that vary greatly from point to point on the table. The vibration, measured in units of root mean square gravity ("gRMS"), varies by as much as 2.7 times from one location to another. Furthermore, the ratio of vibration between axes X/Z, Y/Z, and X/Y also varies greatly from location to location across the same vibration table. If this equipment were used for the ESS process, the fatigue damage imparted to a device tested at one location on the table could be up to 20,000 greater than the fatigue damage imparted to a device tested at another location on the table. Conversely, can it confidently be asserted that a device which receives 20,000 times less fatigue damage has been properly screened to determine if it is free from defects.
Finally, this ESS process equipment supplies most vibration energy at comparatively high frequencies, e.g. above 500 Hertz ("Hz"). It provides almost no vibration energy below 50 Hz. Effective ESS requires vibrating a device at low frequencies simultaneously along each of a device's three axes, i.e., X, Y and Z.
It has been suggested that the observed inconsistencies in the performance of this ESS vibration equipment and its large variations in vibration levels can be remedied by tuning the shaker table. Thus far, elimination of the observed inconsistencies by tuning the shaker table has not been demonstrated. Even if table tuning could eliminate the inconsistencies and large variations, the need for tuning itself poses a problem. Tuning a shaker table for a single device may be possible. However, tuning a table for simultaneously testing several devices at one time in a production environment is impractical. Such tuning would be labor intensive and require skilled technicians to tune and re-tune the shaker table for each successive set of devices.
Based upon the evaluation of presently available commercial equipment, there presently does not appear to exist equipment suitable for the ESS process that is capable of vibrating a device simultaneously along each of its three coordinate axes at uniform and repeatable acceleration at frequencies as low as 50 Hz.
U.S. Pat. No. 3,712,120 that issued Jan. 23, 1973, on an application filed on an invention by Clifton R. Simms and Richard C. Taylor ("the Simms et al. patent") discloses a multi-axis vibration fixture having, in one embodiment, an inclined mounting face to which is secured a rotatable specimen mounting fixture. Selecting an angle for the fixture's mounting face and skewing the mounting fixture about an axis perpendicular to the mounting face permits simultaneously vibrating a specimen along each of its three major orthogonal axes. Furthermore, rotation of the mounting fixture permits vibrating a specimen along other orthogonal axes which are not the specimen's major axes.
The vibration fixture disclosed in the Simms et al. patent is configured for proving a specimen's strength under extraordinary conditions. The vibration fixture was specifically set up to test items of various sizes and shapes. Thus this fixture provides a vibration apparatus with flexibility sufficient to meet most sizes and shapes of specimens to be tested. The Simms et al. patent discloses that it is desirable to have a vibration fixture which can be simply and quickly adapted for a wide variety of different items. Also, the vibration fixture disclosed in the Simms et al. patent was configured, through its rotatable mounting fixture, to minimize or avoid any structural modification in altering test conditions. Therefore, the multi-axis vibration fixture disclosed in the Simms et al. patent permits performing a wide variety of different strength tests with a minimum of effort.
While the vibration fixture disclosed in the Simms patent, and a similar fixture disclosed in U.S. Pat. No. 5,156,051 that issued Oct. 20, 1992, on an application filed by Philip Marshall entitled "Vibration Test Fixture" ("the Marshall '051 patent"), are adaptable for testing specimens of various sizes and shapes, they are ill-suited for the ESS process. Performing ESS in a production environment requires mounting the same sized device over and over again onto the same vibration fixture. Furthermore, ESS is performed not to determine a device's strength, but rather to determine its freedom from defects. As explained previously, such production screening, if it is to be effective, requires repeatable and uniform vibration testing. Furthermore, it is awkward and expensive, if not impossible, to properly perform ESS using a physically massive, general purpose vibration fixture of the type disclosed both in the Simms et al. patent and in the Marshall '051.